There are known following sorts of prior art SAW convolvers, classified according to the structure thereof. The separating medium structure is a structure, in which a slight air gap is put between a semiconductor layer and a piezoelectric layer. By this structure the advantages of each of the semiconductor layer and the piezoelectric layer can be utilized and it has excellent characteristics. However it can be difficult to produce because of the requirement to form the air gap of an order of 100 nm.
By the elastic structure, although the electrode pattern can be easily formed on a piezoelectric substrate, since the elastic non-linearity of the piezoelectric body is utilized, the efficiency thereof as a convolver is not so high.
By the layered structure a piezoelectric layer is formed on a semiconductor substrate and the electrode pattern is constructed further thereon. Since this structure utilizes non-linear characteristics of the capacity of the depletion layer in the semiconductor body, it has an advantage that the efficiency thereof as a convolver is high.
FIG. 4 is a cross-sectional view of a convolver having this layered structure and FIG. 5 is a plan view thereof, in which reference numerals 1 and 2 are input terminals; 3 and 4 are input matching circuits; 5, 5', 6 and 6' are input transducer terminals; 7 and 8 are input transducers; 9 is an output gate; 10 is an output gate terminal group; 11 is a binary tree input terminal group; 12 is a binary tree; 13 is a binary tree output terminal; 14 is an output matching circuit; 15 is a convolver output terminal; 16 is a bonding wire; 17 is a piezoelectric layer (ZnO, etc.); 18 is a semiconductor layer (silicon, etc.); and 19 is an insulating layer (SiO, etc.). The semiconductor layer is made usually of silicon and the piezoelectric layer 17 is made of zinc oxide. The input transducers 7 and 8 as well as the output gate 9 are metallic films made of aluminum, etc. Discussion of one such prior art convolver employing a binary tree is to be found in an article entitled "Small-Aperture Focusing Chirp Transducers vs. Defraction-Compensated Beam Compressors in Elastic SAW Convolvers (IEEE Transactions on Sonics and Ultrasonics, Volume SU-32, No. 5, Sept. 1985, pp. 675-676).
In order to increase the process gain indicating the performance of the convolver, it is required to increase the product B.multidot.T of the bandwidth (MHz) and the delay time T (sec) due to the output gate 9 (the length of the longer side of the output gate being denoted by l, T=l/v, where v represents the speed of the surface acoustic wave and the frequency bandwidth B is supposed to be 3dB). In order to enlarge the bandwidth B, it may be sufficient to design the input transducer so as to have a wide band. Further the convolution output is generated by the output gate 9 and in order to elongate the delay time T, it is sufficient to increase the longer side l of the gate.
Now transmission characteristics indicating the characteristics of the propagating portion between the input transducer 7 and the input transducer 8 will be explained.
The transmission characteristics are indicated in FIG. 6. This is a so-called band pass filter and it can be understood from the figure that a signal frequency region. This comes from the layered structure indicated in FIG. 4 due to the fact that the propagation speed of the SAW varies depending on the thickness of the piezoelectric layer and the propagation frequency. This is unavoidable characteristics so far as normal type transducers are used, unless special transducers are designed.
On the output gate of a convolver having the layered structure indicating these high frequency attenuation characteristics an output signal having a frequency twice as high as that of an input signal is generated and the frequency characteristics of the output signal appear also in the convolver output characteristics as indicated in FIG. 7. The ordinate in FIG. 7 represents F.sub.T (dBm), which is the output in the case where an electric power 0 dBm is inputted through each of the input terminals. Further, this figure shows a state ideally matched with 50 .OMEGA. for every frequency of the output signal. Consequently the greatest value of F.sub.T, which can be outputted, can be known. FIG. 3 is traced also under the same conditions.
In practice, the input portion and the output portion of a convolver should be necessarily matched by means of matching circuits. The working frequency band B (MHz) required for the convolver is determined by the design of the input transducers. However the working band is reduced by this addition of the matching circuits. Further, when the gate length l is increased in order to increase the delay time T, .DELTA.L indicated in FIG. 6 is increased. Consequently .DELTA.F.sub.T is also increased and as the result the working band is reduced.